No.15 coal seam of Pingshang Coal Industry was mining of the lower coal group under pressure. The floor was threatened by the high confined water of the Ordovician Fengfeng Formation. The depth of floor mining failure was the core parameter of water disaster prevention and control. In order to accurately obtain the failure law of floor under different mining widths of working face, FLAC3D numerical simulation was used to establish a three-dimensional geomechanical model of No.15 coal seam. Three mining widths of 150 m, 180 m and 210 m were set to carry out step-by-step excavation simulation, and the evolution characteristics of floor stress field, displacement field and plastic zone were systematically analyzed. The results showed that with the increase of mining width, the failure depth of floor increases gradually. The maximum failure depth of 150 m mining width was 21.0 m, 180 m mining width was 26.6 m, and 210 m mining width was 28.5 m. The failure mode was mainly shear failure, and the middle of the goaf was accompanied by tensile failure. When the mining width increased from 150 m to 180 m, the failure depth increased the most, and the growth slowed down after exceeding 180 m. The research results could provide a quantitative basis for the optimization of reasonable mining width of working face, the depth design of floor grouting control and the safety evaluation of mining under pressure.